The proposed work has the following main objectives: a) The study of malignant behavior in vivo of hybrids derived from the fusion of normal with malignant cells in vitro, or the fusion of two histologically different tumor types; b) The study of the mechanisms regulating surface antigen expression in hybrids derived from the fusion of cells selected for low antigen expression (immunoresistance) and unselected cells with high antigen expression; c) The regulation of C-type virus production and virus determined membrane antigen expression in hybrid cells derived from the fusion of parental cells defined with regard to these characteristics; d) The expression of tumor-associated antigens, induced by an oncogenic virus (other than C-type) in hybrids derived from cells containing and lacking the information for the antigen, respectively; e) Exploration of the possibility that tumor cells may fuse with host cells in vivo and selection of the corresponding hybrids.